Showing papers on "Rheometer published in 1993"

Design and construction of a linear shear stress flow chamber

Abstract: A new paralle plate flow chamber that has a linear variation of shear stress, starting from a predetermined maximum value at the entrance and falling to zero at the exit, has been designed and tested. This is in contrast to the usual rect-angular channel plan which produces a constant shear stress over the entire length. The new design is based on the theory of Hele-Shaw flow between parallel plates. To verify the efficacy of the flow channel, the effect of fluid shear stress on platelet adhesion to a fibrinogen-coated glass surface was tested. The percentage of attached platelets after 5 min of shear stress is shown to be a function of shear stress. With this new flow chamber, cell-cell interactions can be studied efficiently over a wide range of shear stress using a single run at constant discharge.

Measurement of biaxial and uniaxial extensional flow behavior of polymer melts at constant strain rates

Abstract: Biaxial extensional flow behavior of polystyrene and polypropylene melts was measured utilizing the lubricated squeezing flow method under the condition of constant strain rate. The uniaxial extensional flow behavior of these samples was also measured using a Meissner‐type rheometer for comparison with the biaxial behavior. The biaxial stress growth coefficient η B + grows more slowly than the uniaxial stress growth coefficient η E + at long times. Deviation of η B + from the low strain rate asymptote is not so prominent as that of η E +. The deviation of η B + and the upturn of η E + from the low strain rate asymptotes are more apparent in broad distribution samples than in a narrow distribution sample. It is found that η B + decreases with strain rate at long times near steady state. The upturn behavior of η E + at high strain rates in a narrow distribution sample is discussed based on the Doi–Edwards theory and stress relaxation data. The upturn is clearly observed when the strain rate exceeds the contraction (retraction) rate τeq −1, where the contraction time τeq is evaluated from the relaxation data. Consideration on chain stretching in various multi‐axial extensional flows gives a prediction for deviation of stress growth coefficients from the low strain rate asymptotes in uniaxial, biaxial, planar, and ellipsoidal extensional flows.

A specific slit die rheometer for extruded starchy products. Design, validation and application to maize starch

Abstract: A novel in-line rheometer, called “Rheopac”, has been designed and built in order to study the rheological behaviour of starchy products or, more generally, of products sensitive to a thermomechanical treatment. It is based on the principle of a twin channel, using a balance of feed rate between each of them, in order to make local shear rate vary in the measuring section without changing the flow conditions into the extruder. A wide range of shear rate could be reached and measurements were performed more swiftly than with a classical slit die. The viscous behaviour of maize starch was studied by taking into account the influence of the thermomechanical history, which modified the starch degradation and thus led to important variations in the viscosity. Experimental results were satisfactorily compared to previously published models.

A Critical Appraisal of Available Methods for the Measurement of Extensional Properties of Mobile Systems

Abstract: There is now a general consensus in the field of non-Newtonian fluid mechanics that the extensional viscosity of mobile highly elastic liquids can be a very important influence in determining flow characteristics. Depending on the complexity of the flow geometry, it can be at least as important as the more widely measured shear viscosity, and in some cases it may be the more dominant influence. Not surprisingly, this realization has motivated the development of extensional rheometers. The process has been reasonably successful for (stiff) highly viscous liquids like polymer melts, although it must be conceded that even in this case there can be severe limitations on the attainable range of equilibrium strain rates. The problems for mobile systems, like polymer solutions, are more acute and they are of a different nature to those encountered with stiff systems. Basically, the difficulties are associated with the need to generate flows with a well-defined extensional flow field, free of a substantial shear component.

Rheological characterization of stirred yoghurt: viscometry

Abstract: Rheological characteristics of stirred yoghurt, varying in dry matter content, fermentation temperature and composition of bacteria cultures, were evaluated using a Bohlin VOR Rheometer. Flow curves, covering the shear rate range 29–920 s−1, generally indicated the presence of a yield point and pseudoplastic behaviour, but also Bingham plastic behaviour was observed for some of the specimens. Systematic variations in the profile of the flow curves were observed as a result of the applied manufacturing conditions. Flow curves exhibiting pseudoplastic behaviour with a yield stress could not, over this broad shear rate interval, be described satisfactorily by the often applied Casson or Herschel-Bulkley models, and consequently a new one, the QRS-model was introduced. This model contains three parameters and gave a significantly better fit, with R2 values generally above 0.99.

Shear rate thickening flow behavior of semisolid slurries

Abstract: The constitutive flow behavior of semisolid slurries is a strong function of temperature and prior processing history. The history dependence of this constitutive behavior lends itself to modelingvia an internal variable framework that separates the evolution of structure from the flow behavior at a constant structure. We have assembled a computer-controlled, high-temperature Couette rheometer and have characterized the constant structure constitutive behavior of a tin-15 pct lead system from 0.3 to 0.45 fractions solid. The flow behavior at a given material structure is shear rate thickening and not shear rate thinning, as is commonly assumed.

Effect of molecular weight on the rheological behavior of thermotropic liquid-crystalline polymer

Abstract: Using a cone-and-plate rheometer, the steady and oscillatory shear flow properties of the polymer [OPhCOOPh(SO 2 Ph)OCOPhO(CH 2 ) 10 ] n (M w = 33000-45000, M w /M n = 2, T n→i = 161-176°C) were measured in both the nematic and isotropic regions. It was found that (a) η 0 ∞ M 6.5 and negligible N 1 in the isotropic region and (b) η 0 ∞ M 6 and N 1 ∞ M 6.7 in the nematic region, where M is the molecular weight, η 0 the zero-shear viscosity and N 1 the first normal stress difference

Evaluation of blood viscosity at high shear rate with a falling ball viscometer

Abstract: Received 20.12.1992; accepted 16.4.1993) We tested the accuracy and the usefulness of the MT90 (falling ball) viscometer as an alternative to more expensive and sophisticated devices for physiological and clinical investigations. This instruments measures viscosity from the velocity of a ball falling through a syringe filled with less than 1 ml of blood or plasma at a shear rate of 1000 s-1, Le. the range of the so-called 'newtonian' behavior of blood. We postulated that this shear rate allows us to measure RBC rigidity in a completely disaggregated structure. Four indices of RBC rigidity derived from high shear rate viscometry were used: 1J45r (Le. blood viscosity at corrected hematocrit 45% divided by plasma viscosity); 'Tk' (Dintenfass); 'k' (Quemada); 'IJ(RBC), (Breugel and coiL). All were strongly correlated to each other (r=0.98 p<0.01) and easily detected in vitro rigidification of RBCs with (a) ionophore A23187 (10-4M); (b) heating at 56·C during 10 min; (c) 30 min incubation in hypercalcemic-hyperosmolar buffer, when used together with Hanss' hemorheometre. At this shear rate, modifications of blood viscosity induced by changes (within a physiological range) of hematocrit and RBC rigidity were correctly described by both Quemada's and Dintenfass's equations for blood viscosity. 'k' values given by the equation of Quemada at this shear rate correlated with RBC rigidity indices calculated with the Carri-Med rheometer using both Quemada's and Wang's equations. Correlations between these indices of rigidity and those given by the hemomeometre were found only for in vitro experiments of RBC stiffening, while in clinical situations viscometric and hemorheometric indices gave rather different results. The coefficient of variation for viscometric measurements was 2% (plasma viscosity) and 3% (blood viscosity). We conclude that this viscometer gives results consistent with other methods. We suggest that it is useful and accurate for physiological and clinical studies.

Effect of Surface Coatings on Wall Slip of LLDPE

Abstract: The polymer-metal interface in a sliding plate rheometer was modified by applying three fluoropolymer coatings to the plates in order to examine their effect on the wall slip behavior of a linear low density polyethylene. All three coating materials increased the slip velocity markedly. Furthermore, it was found that the critical shear stress for the onset of slip scales well with the work of adhesion of the interface, suggesting that slip is the result of adhesive failure at the interface.

Transient rheological behavior of a thermotropic liquid‐crystalline polymer. I. The startup of shear flow

Abstract: The transient rheological responses upon startup of shear flow were investigated for a well‐characterized thermotropic liquid‐crystalline polymer in the nematic region using a cone‐and‐plate rheometer For the study, an aromatic polyester, poly[(phenyl sulfonyl)‐p‐phenylene 1,10‐decamethylene‐bis(4‐oxybenzoate)] (PSHQ10), was synthesized via solution polymerization in our laboratory The PSHQ10 was found via gel permeation chromatography to have the weight‐average molecular weight of 45 000 relative to polystyrene standards and a polydisperity index of 2, and via differential scanning calorimetry to have (a) a glass transition temperature of 88 °C; (b) a melting point of 115 °C; and (c) a nematic–isotropic transition temperature (TNI) of 175 °C The initial morphology (ie, initial conditions) for the startup of shear flow of PSHQ10 in the nematic region was controlled by first heating a solvent‐cast specimen to 190 °C (ie, to the isotropic region), shearing at a rate of 0085 s−1 for 5 min, and then c

Process rheometers for molten plastics: A survey of existing technology

Abstract: There are many operations in the plastics industry where it is useful to be able to monitor the rheological properties of a melt as it is being processed. Such operations include polymerization, blending, compounding, and reactive extrusion. The design of such instruments poses major challenges, as there is a conflict between the need for rapid sample renewal and the desire for measurement accuracy. In-line types, which are installed directly in the process flow, have a rapid response but must operate in an environment where temperature, pressure, and flow rate fluctuate in response to process upsets. On-line types, which make use of a side stream and gear pump to feed the rheometer, allow for more freedom in the design of the rheometer but have an inherent signal delay due to the time required for melt to reach the rheometer. A variety of rheological sensors have been described in the patent and research literature, each with its unique advantages and disadvantages, and some of these are finding increased use in the plastics industry to improve product quality and process efficiency.

Behavior of a Squeeze Film Damper with an Electrorheological Fluid

Abstract: The flow properties of electrorheological (ER) fluids change with the application of an electric field. These materials have been proposed as smart lubricants. Existing ER fluids are best described by the Bingham model. The Bingham material is described by two parameters, yield shear stress and viscosity. When the shear stress magnitude exceeds the yield shear stress, quasi-Newtonian flow results; otherwise, the material is rigid. For many ER fluids, the yield shear stress is proportional to the square of the applied electric field. In the present study, the Bingham model is applied to the one-dimensional squeeze film damper. A rigid core forms midway across the film, the core thickness being proportional to the yield shear stress. A modified Reynolds equation is obtained, from which the bearing behavior can be predicted. The damper forces are predicted as a function of the eccentricity ratio, and a dimensionless parameter which depends on the yield shear stress. Calculations are performed for a simple ro...

Static shear stress of electrorheological fluids.

Abstract: We have calculated the static shear stress of an induced electrorheological solid for a single-chain structure, double-chain structure, triple-chain structure, and body-centered tetragonal (bct) lattice. When the shear strain is small, all of these four structures prefer slanted configurations which will come back to the original configurations if the load is removed. As the shear strain exceeds a yield point, the structures break into parts which cannot return to the original configurations in a short time. The bct lattice is found to have the strongest shear modulus. The triple-chain structure is weaker than the bct lattice, but much stronger than the single-chain structure and double-chain structure. The single-chain structure has the Peierls-Landau instability if the chain is very long. A double chain is stronger than a single chain if the chains are quite long and the situation is reversed if the chains are short.

Internal viscosity dumbbell model with a Gaussian approximation

Abstract: A noninertial Hookean dumbbell with internal viscosity (or internal friction) is studied in transient and steady shear flows by use of a Gaussian closure on the second moment equation of the configuration of the dumbbell. The model predicts shear thinning for the viscosity and first normal stress coefficients for all values of the relative internal viscosity parameter e. A second Newtonian region is observed for the viscosity. Qualitative, but not quantitative, agreement is found with optically determined orientation angles of polymer coils in steady shear flows for dilute polymer solutions. The model greatly overestimates the amount of relative stretching of the polymer coil in steady shear flow. In startup flows, large, but finite, values of e show shear stress overshoot at high shear rates, and oscillatory behavior at the highest shear rates studied. Transient negative values of the first normal stress difference are also predicted. The maximum in stress is attained at much lower values of strain than for the predictions at small e. The oscillations are shown to be caused primarily by oscillations in the orientation of the polymer coil, rather than by oscillations in the size of the polymer coil. Instantaneous jumps in the shear stress at t=0 are observed in agreement with Manke and Williams. Cessation of shear flows shows a jump in stress in agreement with data on xanthan gum. The decay upon cessation is nonexponential, but does follow the Lodge–Meissner relation. The width of the polymer coil is predicted to go through a maximum during this decay. Also, the addition of internal viscosity to the dumbbell satisfactorally gives a positive asymptotic value for η’−ηs (in‐phase complex viscosity minus solvent contribution) in small amplitude oscillatory shear flow.

Free oscillatory shear measurements : an interesting application of constant stress rheometers in the creep mode

Abstract: A constant stress rheometer in the creep mode was used to perform free oscillatory shear measurements on soft solids. The results obtained are in good to excellent agreement compared to forced oscillatory shear measurement data. Depending on the rheologic properties of the sample and the moment of inertia of the rotating device of the measurement system, free oscillations are suited to confirm or supplemented forced oscillatory measurement results.

Lubricated flow elongational rheometer

Abstract: A novel method and apparatus for measuring elongational viscosity are disclosed. It is shown that in a hyperbolic or semi-hyperbolic die geometry with lubricated flow, the elongational viscosity of a fluid can be obtained from pressure drop--flow rate data. Experimental data for polypropylene as core and polyethylene as skin material, and the calculated extensional viscosities are presented. Alternative embodiments of the novel elongational rheometer are described, including alternatives suitable for biaxial flow.

Flow behavior of wheat flour-water dough using a capillary rheometer. I: Effect of capillary geometry

Abstract: Cereal Chem. 70(l):59-63 A capillary extrusion rheometer was employed for detailed investigation coefficient of 2,395 Pa-sec 03 4 . The flow curves, corrected for end effects of the flow behavior of wheat flour-water dough. Dough was extruded and for effect of die diameter on shear rates, were independent of capillary at ambient conditions through capillaries of different lengths and diamdimensions. The capillary rheometer technique was found to be a reliable eters. In the shear rate range of 9-5,000 sec-1, the dough exhibited shear and repeatable method for determining flow parameters of viscous mathinning with an average flow behavior index of 0.34 and consistency terials such as dough. As with most foods, rheological properties are important in breadmaking. The final quality of the bread can be related to the rheological properties of its flour dough, which explains the amount of research being conducted in this area. Researchers have addressed different aspects of dough rheology and have described the methods used to evaluate the rheological properties (Bloksma 1975; Hibberd and Parker 1975; Matsumoto et al 1975; Matsuo and Irvine 1975; Rasper 1975; Bushuk 1985; Dick 1985; Faubion et al 1985; Hoseney 1985; Nagao 1985; Dreese et al 1988a,b; Fitzgerald et al 1988; Refai et al 1988). Their work concentrated on the qualitative evaluation of flours. Instruments such as the farinograph or mixograph were used to obtain optimum 'Published as journal series 9881, Agricultural Research Division, University of Nebraska. 2 Use of a company or product name by the U.S. Department of Agriculture does not imply approval or recommendation of the product to the exclusion of others that may also be suitable. 3 Packaging engineer, Westreco, Inc., St. Joseph, MO; professor, Biological Systems Engineering, University of Nebraska, Lincoln; research leader, USDA-ARS, Beltsville, MD, respectively. @1993 American Association of Cereal Chemists, Inc. mix time and absorption data. These instruments provide useful information about dough, but they cannot generate numerical data to characterize flow behavior. In automated bakeries, knowledge of dough viscosity plays an important role in production control and equipment design. Dough viscosity may relate to the quality of the baked product and may control that quality in some instances. On-line methods to sense dough consistency require knowledge of dough flow behavior over a wide range of flow conditions. Dough can be pumped from the point of mixing to the fermentation chamber and then to the oven, but suitable pumps cannot be designed or specified unless the dough's rheological parameters and flow behavior properties are known. None of the studies referenced above give the basic information on flow behavior needed for engineering design. Basic information on flow behavior of wheat flour-water dough based on a capillary extrusion rheometer was reported by Sharma et al (1990). They found it was possible to describe the flow behavior of dough using the capillary rheometer results. Because only one size capillary (3.21 mm diameter) with different lengths was used, the reliability of the technique needs verification at other capillary diameters. Therefore, this study was undertaken to investigate the effects of capillary geometry on the flow behavior of wheat flour-water dough. Vol. 70, No. 1, 1993 59 MATERIALS AND METHODS Rheometer The capillary extrusion rheometer consisted of a cylinder-piston arrangement (Sharma and Hanna 1992). The stainless steel cylinder had an inside diameter of 37 mm and a wall thickness of 17 mm. A close-fitting plunger was constructed from a solid brass rod. The working length of the plunger was the same as the 100-mm height of the cylinder. One end of the plunger had a threaded portion that could be screwed into a coupler, which, in turn, was screwed to the cross-head of a Universal Testing Machine (UTM) (model 1123, Instron Corp., Canton, MA). The capillaries were hollow brass tubes. One end of each tube was soldered to a threaded stainless steel section that could be screwed into the threaded hole at the bottom of the cylinder's supporting collar to be flush with the inside surface. The inside diameter of the stainless steel supporting collar was slightly bigger than the outside diameter of the cylinder. The height of the supporting collar was 56 mm to avoid buckling due to excessive pressure exerted on the cylinder side walls. Three set screws were used to secure the cylinder in the supporting collar. The supporting collar was held vertically on a flat plate and the entire assembly was supported by a heavy steel stand. The complete unit was positioned under the cross-head of the UTM so that the brass plunger would smoothly slide down inside the cylinder containing the dough and force-extrude the dough through the various capillary sizes. Dough Preparation Bakery flour was obtained from ConAgra (Omaha, NE). The moisture content of flour was determined to be 12.6% on a wet basis (method 44-15A, AACC 1983) of drying 3 g of flour at 130'C for 1 hr in a convection oven. Bakery flour was tested in a 10-g mixograph for optimum mix time and absorption. These were found to be 3 min and 64.5 g of H20 per 100 g of flour (on a 14% absorption basis), respectively. Subsequently, larger quantities of dough were made by mixing 100 g of flour with 64.5 g of distilled water for 3 min in a pin mixer (National Manufacturing Co., Lincoln, NE). Rheometer Operation The inside wall of the cylinder was lightly lubricated with corn oil. Small pieces of dough were placed in the cylinder and pushed down using a wooden rod. When the cylinder was completely filled, a lightly lubricated plastic sheet was placed on top of the sample and hand-pressed to squeeze out the excess dough and to level the dough surface. A capillary tube of known dimensions was then screwed in place. Different lengths of capillary tubes with inside diameters of 3.2, 4.0, and 5.6 mm were used to achieve various combinations of length-to-diameter ratios (L/D ratio). These ratios are given in Table I. Each capillary was first used at its maximum length. After ensuring that sufficient data were collected using one length, the capillary tubes were cut to obtain shorter lengths. After the cylinder was placed inside the supporting collar and secured, the whole assembly was placed under the cross-head of the Instron, to which the brass plunger had already been attached. The plunger was moved down at preselected speeds of 5, 10, 20, 50, 100, and 500 mm/min to extrude the dough through the capillary tube. The extruded dough was collected TABLE I Length-Diameter Ratios for Various Capillaries

The Rheology of Lubricants at High Shear Rates

Abstract: A modified torsional Kolsky bar is used to shear a thin annular layer of lubricant held between concentric cylinders. The shear rates obtained range from 4×10 3 to 4×10 4 s -1 ; the duration of the shear pulse is about 400 microseconds. The shear stress history and the corresponding shear-rate history are deduced using standard torsional Kolsky bar techniques. This experimental technique provides the capability of measuring the shear stresses sustained by fluids at very high shear rates, while the short test duration ensures a minimal rise in temperature due to the associated shear heating

Method of measuring rheological properties and rheometer for carrying out the method

Abstract: In a method of measuring rheological properties of a sample, an oscillation system containing the sample is caused to perform free oscillations, and the damping caused by the sample is determined, the rheological properties of the sample being possible to determine by means of the damping. By this method, rheological properties can be determined for samples of a small volume and/or low viscosity. The method is especially adapted to measure body fluids. A rheometer for carrying out the method is also disclosed.

Investigations of the rheological behavior of rubber-carbon black compounds over a wide range of stresses including very low stresses

Abstract: The rheological properties of gum and carbon black compounds of an ethylene-propylene terpolymer elastomer at 20, 30, 40, and 50 vol.% carbon loadings have been investigated over a very wide range of stresses and shear rates. This was accomplished using (1) a constant shear stress creep instrument, (2) a rotational rheometer and (3) a capillary extrusion rheometer at higher shear rates. The stresses used in the studies included magnitudes leading to shear flow and stresses below the yield value, where rubber compounds exhibit only finite deformation. Much attention was given to measurements of creep and strain recovery at low stresses. The magnitudes of yield stresses obtained from these low stress and recovery experiments are significantly lower than those obtained using standard extrapolation to zero shear rate of higher stresses from rotational and capillary instruments.

Slip and no-slip squeezing flow of liquid food in a wedge

Abstract: Squeezing flow in a wedge simulates a number of practical processes, e.g. lubrication, coating and the sensory evaluation of liquid foods. This paper reports analytical approximate solutions for both slip (or lubricated) and no-slip squeezing flow of liquid food in a wedge, in which the power law fluid model was used. The solutions do not seem to be more complex than that for squeezing flow between two parallel disks and may be used as a variation of the psycho-physical models of in-mouth viscosity and food spreadability. Alternatively, a rheometer for testing liquid foods may be developed according to the equations described in this paper.

Rheometer with flow diverter to eliminate end effects

Abstract: A rheometer comprises a thin-walled bob having two ends, adjacent at least one of which ends there is disposed a respective flow diverter plate to eliminate adverse end effects caused by swirling fluid in the rheometer.

Slit rheometer studies of wheat flour dough

Abstract: Rheological properties of bread dough having different levels of added gluten (0, 2.5, 5.0 and 7.5%) were measured. The dough was forced through a slit die attached to a cylindrical container. The material functions of interest included steady shear viscosity, first normal stress coefficient in steady shear flow derived from hole pressure measurements and planar extensional viscosity as estimated from entrance pressure drop measurements. These rheological properties were correlated to the loaf volume of the baked bread. Results indicate that the first normal stress coefficient and planar extensional viscosity were affected by the gluten content of the dough and correlated well with the loaf volume. The entrance pressure drop increased with increasing apparent shear rate but entrance correction was found to decrease with apparent shear rate. Water had a greater effect on the rheological properties than gluten content. The Trouton ratio was rather large and ranged between 850 to 2000 at a strain rate of 1s-1 and decreased with increasing extension rate for most cases.

Experimental evidence of slip development in capillaries and a method to correct for end effects in the flow of xanthan solutionsa)

Abstract: Flow experiments using a capillary rheometer with 0.2% xanthan aqueous solutions were carried out to give experimental evidence of the dependence of the slip velocity on the geometrical parameters of the rheometer. The Mooney method was used to evaluate the slip velocity Vs. It was found that Vs is an increasing function of the wall shear stress and also of the length to diameter ratio L/D not considered in the Mooney theory. That is, Vs=Vs(τw,L/D), although, for each τw value, Vs becomes independent of L/D at large L/D. In addition, a method to determine the excess pressure drop due to end effects is outlined and experimental results presented. The differences between this method and the one by Bagley are highlighted.